Amyloid beta (Abeta) is a fibrillar component of amyloid in senile plaques and cerebral blood vessels in Alzheimer's Disease (AD), and a soluble peptide (sAbeta) normally present in body fluids. The origin of Abeta deposited in brain and blood vessels in AD is uncertain, and whether or not sAbeta is the direct precursor of amyloid it is not known. The current concept states that a key question in amyloidogenesis is to determine what factors are responsible for the conformational alteration of sAbeta into its fibrillar form. The unifying hypothesis of this proposal is that the blood-brain barrier (BBB) may play an important role in cerebrovascular and brain amyloidogenesis by regulating transport of sAbeta and two amyloid-associated proteins shown to bind sAbeta, apolipoproteins E (apo E) and J (apo J). In support of this hypothesis we have recently provided the evidence that a synthetic peptide homologous to major form of circulating sAbeta, sAbeta(1-40), is taken up into the brain via specific mechanism, and possibly as an sAbeta-apo J complex. In contrast, blood- brain transport of sAbeta-apo E was negligible. Cerebrovascular permeability to apo J was significant, while apo E had a limited access across the BBB, indicating that the apo E found within the brain is produced locally. Our pilot in vitro experiments demonstrated that sAbeta(1-40) binds to apo J with the affinity that is about 10 times higher than for apo E. Studies conducted in vivo indicated that sAbeta(1- 40) binding to apo J and apo E results in remarkable 114-fold difference in barrier permeability to their respective complexes. In the present proposal, we plan to characterize the mechanisms of sAbeta transport at the BBB in greater detail. Specific hypotheses to be tested are: 1. Binding of sAbeta to apo J and apo E isoforms 2, 3 and 4 modulates cerebrovascular permeability to sAbeta; 2. sAbeta peptides are transported across the BBB by a specific transporter and/or receptor; 3. Transport of sAbeta complexed to apo J and apo E isoforms incorporated into high and very low density lipoprotein particles, i.e., HDL and VLDL, may be mediated by HDL and/or low density lipoprotein (LDL) BBB receptors; 4. Aging alters transport mechanisms at the BBB predisposing to accumulation of sAbeta and amyloid-associated proteins in cerebral microvessels and brain parenchyma. The guinea-pig with the Abeta sequence identical to humans will be used. The affinity of sAbeta binding to apolipoproteins will be determined in vitro. Transport studies will be performed in the in situ vascular perfused brain, in conjunction with the capillary depletion technique, radio-HPLC end immunocytochemical analysis. The putative sAbeta transporter and/or receptor will be isolated from BBB plasma membranes. Proposed studies should help us understand transport mechanisms of sAbeta at the BBB, with an ultimate goal to develop strategies that may reduce accumulation of sAbeta and amyloid-associated proteins in brain and cerebral microvessels, thereby decelerating and/or preventing conformational transformation of sAbeta and its potential cytotoxic effects.